This invention relates in general to welding and more particularly to welding tubes to tubesheets.
In the typical tube-type heat exchanger, the tubes extend between tubesheets to which they are welded. Thus, one fluid may pass through the tubes while another circulates around the exterior of the tubes in the region between the two tubesheets, and heat will transfer through the tube walls from the one fluid to the other. Some fluids, when in the vapor phase, have a tendency to enter crevices and leave deposits. Any tubesheet which is exposed to this type of fluid should be welded to its tubes where the tubes enter the tubesheet, that is at the back face of the tubesheet. This eliminates crevices. In this connection tubes which have been welded to the front faces of their tubesheets have, on occasion, collapsed due to the build up of deposits in the crevices between the walls of the tubesheet holes and the tubes.
Heretofore, the traditional approach for welding a tube to the back face of a tubesheet has been to align the tube with its hole in the tubesheet, preferably by inserting the end of the tube a short distance within the hole, and then by means of an electrode inserted through the hole melting the tube and the surrounding portion of the tubesheet, so that the two join together at a weld along the back face of the tubesheet. In order for the weld to achieve full penetration the power must be quite high, but this makes the weld difficult to control. Sometimes a burn-through occurs, but more often the weld metal tends to flow in an undesirable manner. For example, when the weld is made with the tube extended horizontally, the molten metal will tend to flow toward the bottom of the tube so that the finished weld is thickest at its bottom. For this reason, many welders weld with reduced power, which often does not provide adequate penetration, or else they fabricate only with the tubes oriented vertically so that all welding is done in a down-hand position. The latter is not practical where the overhead clearance is low.
Aside from the difficulties of controlling the weld, an inert gas must circulate both within the tube and around the exterior of the tube at the back face of the tubesheet. The former presents no difficulty, for the tube itself confines the gas and prevents it from dissipating. This does not hold true at the back face. Also, the inert gas adds an additional expense to the welding procedure.